1. Field of the Invention
The present invention relates to the field of harvesting apparatus, and more particularly to apparatus for sensing the maturity of vegetables and/or the existance of contaminants in growing and harvested food stuffs.
2. Prior Art
Certain food stuffs such as wheat are harvested based upon the average maturity of the crop or field. Other food stuffs, however, often are selected for harvesting at a particular picking based upon their size, weight, etc. One such food stuff, by way of specific example, is lettuce, wherein the chief criteria for appropriate maturity is the density of the head. In that regard, different heads, even in the same row and thus supposedly subjected to the same growing conditions, will mature at different times so that the best crop yield will be obtained through the use of multiple pickings, selecting only the appropriately mature heads each time. While the size of the heads are visually perceivable and could be optically detected automatically, size for certain food stuffs such as lettuce is not an adequate indicator of density, and thus is not an appropriate basis for determining the appropriate picking time. Thus, in manual harvesting, the field hand will normally lightly squeeze the lettuce head to determine suitability for picking, the experienced field hand exhibiting considerable expertise in the judgements made.
It has been recognized that mechanical (automatic) lettuce pickers could be used to harvest a lettuce crop at a substantially lower cost, provided some appropriate method of automatically testing the lettuce head for density could be implemented so that individual picking decisions could be made as the harvesting machine is moved along the lettuce rows.
One method and apparatus for automatic measurement of size and density of produce is disclosed in U.S. Pat. No. 3,594,579. That system utilizes Americium 241 radioactive isotope as a source of gamma rays for size and density selection. The gamma ray source is collimated into a pencil beam which passes through the lettuce head and is detected on the other side thereof by a photomultiplier. By observing the count rate decrease at the center of the head, an attenuation criteria for a fully mature head may be established. That system also contemplates the measurement of a head diameter based upon the distance along the row over which the radiation attenuation was substantial because of the existance of the head between the source and detector.
The gamma ray isotope system just described is relatively simple, but has certain very serious drawbacks. In particular, approximately 30 millicuries of Americium 241 is required, representing a fairly large amount of the very toxic isotope. Although the source is sealed, unexpected accidents could result in the loss of containment which would pose a very serious health hazard in food stuffs, as gamma ray emitters such as Americium 241 are associated with alpha particle emitters and tend to be bone seekers if ingested. As a result, controlling agencies are quite worried about granting licenses for gamma ray emitters for use in the crop fields because of the possible consequences of an unexpected event.
P. A. Adrian and D. H. Lenker have used a maturity sensing system having an x-ray generator as an x-ray source and a pair of silicon photodiodes as detectors. The detectors are arranged such that one photodiode sees an unattenuated beam while the other sees a beam attenuated by the presence of a lettuce head. The difference in the current detected in such a system can be correlated to maturity for the produce being tested. (A more complete description of that system may be found in a paper entitled "A Selective Crisphead Lettuce--Harvest System" by D. H. Lenker, et al., presented at the 1972 Annual Meeting of the American Society of Agricultural Engineers, Hot Springs, Arkansas, as paper number 72-146). The foregoing system operated well in terms of accuracy of the measurements but was subject to the difficulties commonly encountered with x-ray generators in general. In particular, x-ray tubes have a limited and unpredictable life, are shock sensitive and have (with their auxiliary high voltage power supply) failure modes which can cause excessive radiation output. In addition, their output can vary with temperature, supply voltage and life, requiring the reference channel hereinbefore referred to to normalize the unattenuated beam. The particular generator used for those tests was a converted World War II surplus dental x-ray unit no longer available on the market. In such a system the total flux of x-rays is quite high due to the lack of sensitivity in the detector used. In addition, the pulse nature of the x-ray beam requires additional circuit complexity with the attendant cost and maintenance problems. Of course, that system, as well as the gamma ray system hereinbefore described, had no provision for linearizing the density versus voltage output of the electronics, resulting in a signal characteristic which is difficult to interpret by the untrained eye, and not the most desirable for automatic decision making purposes, as it is very awkward to present to untrained personnel for changing the acceptance level in the field. This problem may be particularly illustrated by noting that the attenuation of a gamma or x-ray beam passing through any material may be expressed as EQU R=R.sub.o e.sup.-.mu..rho.x
Where
R is the attenuated beam in photons/sec. PA1 R.sub.o is the unattenuated beam in photons/sec. PA1 .mu. is the attenuation coefficient of the material in cm.sup.2 /gr PA1 .rho. is the density of the material in gr/cm.sup.3 PA1 x is the thickness in cm
Thus if the .mu. is constant and the electronics respond linearly to the attenuated count rate, the output increases proportionally to the reciprical of the density-thickness product, i.e. EQU output voltage.varies.1/e.sup..rho.x
If, on the other hand, the electronics could extract the log of R (as in the present invention), the output signal would be EQU V.sub.1 .varies. log (e.sup.-.mu..rho.x)=-.mu..rho.x